It was 4-6" deep....I love doing everything twice ...I should have known better....all done now! I used the quickrete primer, painted it on the old concrete, drilled 1/2" holes and screwed in some 1/2" galvanized support hooks (7 of them), mixed up 3 bags of quickrete 1101, not super wet, and packed it back in. It isn't beautiful, but it's getting covered by the shower floor!

Yes, it said you can paint it on with a brush, or mix it with Portland cement for a slurry. I painted it, and let it dry while I was mixing the cement. It said it was to be used to bond the new concrete with the old...please don't tell me it's crap...and the sides of the trench we're not smooth, there should be plenty of purchase for the new concrete to bite in to.

I mentioned the primer only because we usta use Emer's Glue-All for that application years ago and it was usually a lot less expensive than proprietary primers. It works, but these days I much favor a slurry of thinset mortar for that application. Was just curious.

Your Kerdi drain should not fit inside a 4" PVC pipe. It will fit inside a 4" PVC pipe coupler.

In any case, you're not gonna get any deck mud down in that hole beside the drain flange and it doesn't matter if the coupling is left in place. I like to toss a handfull of thinset mortar into a gallon can of deck mud and add a little water for the portion that will be packed under the drain flange. I favor the edge of a wood float for packing the mud under there.

The coupling shoulda been left in place, Joe. Now the hole in the slab is larger than it needs to be, but you still shouldn't have any problem so long as you pack your mud under the rest of the flange.